I.A. Environment of Interest
Engineering is based on an understanding of certain physical laws and application of these laws in designing apparatus which will efficiently control the movement of matter and energy. Thermodynamics and heat transfer are two key branches of engineering.
Thermodynamics relates to the relationships between energy and matter, particularly, the relationships among temperature, density, pressure, enthalpy, entropy, etc. Traditionally, thermodynamics has addressed the study of heat engines and refrigeration apparatus.
Heat transfer relates to the modes of heat transfer and methods of predicting heat transfer.
Insulation is a common type of engineering material which is used to control heat transfer and it is often used with a thermodynamic device such as a heat engine or a refrigerator where it is considered to be physically associated with the device, that is, integrated physically to fit and enclose the device. Insulation is not normally integrated into the thermodynamics of the device, that is, made part of the thermodynamic cycle to thereby improve the cycle.
A good thermal insulation system: 1) limits heat transfer by conduction, that is, heat transfer by random molecular motion within a material where the molecules do not move appreciably from a certain point, 2) minimizes heat transfer by convection, that is, physical transport of fluid molecules which carry thermal energy with them from one place to another, and 3) limits thermal radiation.
A good thermal insulation system may actually involve the transfer of significant amounts of heat by controlling where the heat is transferred.
Conduction and convection are normally limited by the use of a barrier layer of material which has a low bulk thermal conductivity. Such material is often porous wherein the cavities in the material are filled with air. Fiberglass, rock wool and asbestos are examples of such material wherein the cavities in the material are interconnected and filled with air (or a specific gas mixture) while wood and styrofoam plastic are examples of materials wherein the cavities in the material are typically filled with air (or a specific gas or gases) and the cavities do not communicate with each other.
Whether the cavities are interconnected or not, heat transfer through these materials by simple conduction through the solid material of the insulation is hindered by the thin direct thermal conduction paths presented by the length of the fibers or by the cavity walls of the material while the fibers or closed cells also inhibit convection of the gas which fills the material. Gases as a class have the lowest thermal conductivities known so that the thermal conduction of the gas in the bulk insulation sets a lower limit to the insulating value of an insulator. An evacuated space may be the basis for still better insulation but such evacuated insulation systems are expensive in design and manufacturing effort.
In many situations, thermal radiation is of limited significance and is considered to be intercepted and controlled by the usual insulation materials.
I.B. General Definitions of Terms
I.B.1 System, Control Volume
Classical thermodynamics studies the interrelationships among heat, work and matter particularly in relation to "thermodynamic cycles". This matter is almost always a fluid and is usually a gas.
Thermodynamic cycles are defined in terms of the "states" of matter in a "system" undergoing "processes": these terms are described or defined in thermodynamic texts such as Classical Thermodynamics by Van Wylan and Sonntag, John Wiley & Sons, 3rd Edition, Eng/Sl version, Copyrighted 1986. Briefly, a process is the path or succession of states through which a system passes. A thermodynamic state is identified as the state or condition of a quantity of matter as determined by temperature, entropy, enthalpy, density, pressure, etc.
Such texts also define a "system" and a "control volume" and set forth both how to define a control volume and how to use a control volume in analyzing energy, mass and momentum flux into and out of a control volume. Very often, the boundary of a control volume is selected to follow the surface of an element such as a wall, that is, a physical solid surface having a locally defined tangent surface or plane. Reference should be made to such texts for more detailed explanations of control volumes and related concepts.
In classical thermodynamics, combustion of a fuel and air is considered in a first approximation to be a method of heating the air and this understanding will be followed herein unless otherwise specified.
I.B.2. Sweep Insulation, Isolation
In addition, "sweep isolation" or "sweep insulation" are defined herein to be isolation or insulation of a region in space (an "isolated region" or an "insulated region") to minimize or effectively block loss of a diffusible quantity to or from an ambient environment by the movement of matter through which the diffusible quantity is transported by diffusion. The diffusive process includes both convection and/or conduction when thermal energy is the diffusible quantity. The moving matter through which the diffusing quantity passes is conveniently a fluid while the diffusible quantity is usually "heat" or "cold": the symmetry of the governing equations allow both to be considered. The motion of the moving matter may be at a constant velocity or a varying velocity. Indeed, there may actually be velocity reversals but there will be an average velocity either toward or away from the isolated region.
I.B.3. Enthalpize, Enthalpizer
In classical thermodynamics, heat exists only when there is a transfer of thermal energy. Unfortunately, the verbs "heat" and "cool" define the direction of energy transfer. Thus, to say that some water is heated indicates that the temperature of the water is increased. If the water is cooled, the temperature is decreased. In both cases, the direction of heat transfer is implicitly defined by which word is used.
There is an archaic word "attemper" which means "to modify the temperature of: make (as air) warmer or colder" (Webster's 3rd New International Dictionary, copyright 1986). However, this term is does not have any apparent technical meaning and thus is ambiguous with respect to whether, for example, an increased temperature is due to adiabatic compression or heating.
The processes which are described by the equations relating to heating, cooling, compression and expansion are more generic than permitted by the English language. When the equations are considered, it is obvious that heating and cooling are the same process which differ only in the mathematical signs ("+" or "-") of the parameters.
The process of changing the enthalpy of a material generally is apparently unnamed in classical thermodynamics. In classical thermodynamics, the enthalpy of a single phase material is a function of the constant pressure heat capacity of the material, the mass of the material present and the temperature of the material. Thus, an enthalpizing process will result in the change of the temperature of a material.
It is thus convenient to define herein a technical term "enthalpize" which refers to the process of changing the temperature of a material such as a fluid by: 1) heat transfer between the fluid and a second heat source or cold sink without regard to whether the material is being heated or cooled or 2) increasing the temperature by an adiabatic process such as compression or decreasing the temperature by an adiabatic process such as expansion or 3) any combination of these processes.
"Enthalpize" is defined as "changing the enthalpy of a material" and refers to any of the processes which include heating (adding heat), cooling (removing heat), compressing (adding energy by means of work) and expanding (removing energy in the form of work) a material, either singly or in combination. Depending on the particular use, any of these terms may be used in place of the term "enthalpize" to thus be more specific and define particular operations.
An "enthalpizer" will comprise apparatus which enthalpizes a material. Since enthalpy for a fixed mass of fluid is a function only of temperature, an enthalpizer will effect a change in the temperature of material on which it acts. Within this definition, "enthalpize" includes changing the temperature of an material such as a beverage which is placed in a refrigerated space in which case the refrigerated space (delineated by the walls of the space) is the enthalpizer. Enthalpizers will include steam boilers, gas heaters, compressors, turbines, expansion motors, etc.
Depending on the context, "enthalpize" may mean any of the following either singly or in combination: heat, cool, compress (but excluding perfect isothermal compression) or expand (but excluding perfect isothermal expansion). Perfect isothermal compression and expansion do not involve a change in the temperature of the fluid undergoing the volume change.
"Enthalpize" and all of its forms (enthalpizer, pre-enthalpize, pre-enthalpizer) represent the equivalent form of the word for which it is a generic form. Thus, enthalpizing one end of a column of air contained within a perfectly thermally insulated and sealed tube may mean that only one end of the air column is enthalpized or that the entire column is enthalpized. If the enthalpizing process is compression or expansion, then, within the context of a time frame allowing pressure/expansion waves to bring the column into equilibrium, the entire column is compressed or expanded. If enthalpize represents heating or cooling, then the time required to equilibrate the column is likely to be very long so that, as understood in context of periods of time shorter than required for equilibration, the column will be enthalpized only at the one end. (Of course, any volume change represented by the heating/cooling will be communicated rapidly at the speed of a compression/expansion wave.)
The term "enthalpize" is an awkward construct but it is based on the root word "enthalpy" and emphasizes the energy change associated with enthalpizing a material. As will be set forth hereinbelow, the pre-enthalpizing of the compressed gas before it is introduced into the enthalpizer is accomplished by means of elements located about and in thermal communication with the enthalpizer, it being immaterial whether the enthalpy transfer used for enthalpizing (heat used for preheating or "cold" used for precooling) comes from elements in the enthalpizer which are more directly involved in effecting a temperature change by heat transfer or combustion (such as but not limited to a heater or burner) or effecting temperature change by adiabatic expansion (such as but not limited to a compressor or turbine).
Similarly, there is apparently no single word that encompasses the meaning "change-the-volume" of a fixed quantity of matter such as a gas or a mixture of a gas and a liquid. The words "compress" and "expand" define specific operations included under "change-the-volume ". Possible words include: 1) (from electronics) "compand" which refers to data compression and expansion as a sequence of operations which together restore the original form of the data and 2) "densify" which is rooted in the word "density" which indicates a quantity ("density" does not imply high or low, increasing or decreasing density but a measured quantity). However, "densify" is limited in its meaning to increasing the density of a material and is essentially synonymous with compress.
An "adiabatic enthalpizer" is defined as being an enthalpizer comprising apparatus to change the density of a compressible fluid with a concomitant change in the temperature of the fluid wherein such apparatus is commonly regarded as being adiabatic in a first approximation or in preliminary engineering analysis. Thus, a piston and cylinder used as a motor or a compressor, an axial or centrifugal compressor or an axial or centrifugal turbine, a steam expander or turbine, a vessel in which a gas such as ammonia is absorbed or desorbed by a liquid such as water (the total volume of fluid as water and ammonia undergoing a change), a diaphragm compressor or expander, etc., would all be "adiabatic enthalpizers" These listed examples are all characterized as apparatus to transfer work energy into or from a fluid thus changing the enthalpy of the fluid (based on conservation of energy) in a thermodynamically reversible process (to a first approximation). (It will be noted that these are studied in thermodynamics first in terms of ideal devices wherein the process undergone by the fluid is considered to be adiabatic and that heat transfer across the boundary or wall of these devices may be acknowledged but detailed analysis is not attempted.) In addition, the definition is to include a Joule-Thompson expansion throttle valve.
A work coupled adiabatic enthalpizer wherein is an adiabatic enthalpizer work is absorbed from or imparted to a fluid which is being enthalpized in the adiabatic enthalpizer. The work will account for some portion of the enthalpy change of the fluid in the work coupled adiabatic enthalpizr.
It will be noted that the combination of an adiabatic enthalpizer in combination with an electric heater, a boiler, a burner, heat exchanger or other device for heating or cooling the material which undergoes a change of state in the adiabatic enthalpizer is to be considered to comprise an adiabatic enthalpizer. A simple burner, or other heater apart from use in combination with an adiabatic compressor or expander (more generally, a volume changer) is not considered to be an adiabatic enthalpizer.
A similar implicit hierarchy may be observed in terminology more commonly used. Specifically, a piston and cylinder having means to heat a compressed gas contained therein prior to a work-producing expansion of the gas is referred to as a motor, engine or expansion motor, but, except under unusual circumstances, not as a gas heater.
It will be understood that physical embodiments of adiabatic enthalpizers will have heat transfer across the boundaries which enclose the particular device under consideration so that reference to an "adiabatic enthalpizer" identifies a class of devices rather than specifying the characteristics of physical embodiments of devices taken from this class.
There is some latitude in how the actual volume change and heating and/or cooling in an enthalpizer may be obtained. For example, the heating and expansion may take place within a single variable volume space defined by a piston and cylinder. Or the fluid heating may take place in a first space or chamber after which the heated fluid is transferred to a variable volume space such as a piston and cylinder. If the fluid heating is obtained by combustion, then multiple sequentially filled and combusted combustion chambers may sequentially feed a single variable volume space. The variable volume space may be obtained by almost any recognized gas expansion motor. For purposes of this paragraph, fluid heating by combusting the fluid with a fuel is equivalent to heating the fluid by the transfer of heat into the fluid from outside of a heating space. Such external heating is intended to include heating by conduction through the walls of the heating space, electric heating elements in the space, etc.
The working space or working volume of a piston and cylinder device will be that first space or volume confined between the piston, the cylinder head and the cylinder walls and any secondary spaces which are at any given instant in free communication with the first space or volume. Since inertial effects associated with rapid fluid flow may effectively isolate one volume of fluid from another (a shock wave isolates the portion of a fluid upstream of the shock from changes taking place downstream of the shock), "free communication" is intended to suggest that pressure changes experienced at one location in a volume of fluid are freely communicated throughout the working volume.